I don't understand this chem sentence...?

Question

This is pretty self explanatory, but it's like the concept I don't get.

Electrons fill in energy levels and orbitals starting with the one that requires the least energy and progressively move to those levels and orbitals that require increase amounts of energy.

Answer 1

When you fill a container, the material first goes to the bottom, then starts to fill from there until finally it reaches the top, yes?  It happens that way because the lowest energy state is at the bottom of the container.  Once the bottom is full, more material can't occupy the same space, so it is forced to be at a higher level, but still goes to the lowest level available.  What happens in the atom is analogous to this.

When you fill a container, the energy levels are defined by the height in the container.  The situation is a little different in an atom, and a little more complicated.  Instead of height above the center of the earth, average distance from the nucleus and angular momentum are primarily what matter.  Instead of physically not being able to occupy the same location, there are the exclusion rules (2 electrons per orbital).

---------EDIT---------

The energy levels wrt average distance from the nucleus are simply defined by coloumb's law.  Closer = lower energy, farther away = higher energy.  But angular momentum matters, too.  The s orbitals have the least angular momentum, so they are lower energy than the p-orbitals, which are, in turn, lower energy than the d-orbitals, and so on.  So for any given energy shell, the s-orbitals will fill before the p-orbitals.  

It starts off simple for the first three levels, then gets messy.

More info on s-p-d-f etc:  http://www.chemguide.co.uk/atoms/propert...

Answer 2

The fill orbitals closer to the atoms since protons attract electrons. Then they move further out from the atom in orbitals. They fill s, p, f, d and so on... Which position do you think would be more favorable? Inner or outer?